Translation in eukaryotes is initiated following recruitment of the 40S ribosomal subunit, which can occur via the m7G cap-structure, a modified nucleotide found at the 5′ ends of mRNAs, or through cap-independent mechanisms. The latter has been termed internal initiation of translation and can be mediated by various mechanisms. Following recruitment by either mechanism, the 40S ribosomal subunit moves to an initiation codon, the 60S ribosomal subunit joins, and peptide synthesis begins. Sequence elements contained within non-coding segments at the 5′ and 3′ ends of mRNAs can affect the efficiency of translation initiation. Sequence elements that can enhance the translation of a cap-dependent mRNA have been termed translational enhancer elements (TEEs).
Some TEEs enhance translation initiation by a mechanism that involves base pairing to the RNA component of 40S ribosomal subunits, the 18S ribosomal RNA (rRNA). Some TEEs can also function as ribosomal recruitment sites, facilitating internal initiation of translation; however, most internal ribosome entry sites (IRESes) do not function as TEEs.